Commonly, in an electrophotographic copying apparatus of the wet developing type, photosensitive material or copy paper, for example, wound on a rotatable spool is fed, by a plurality of rollers, into a developing device which contains therein an organic developing solution including toner particles for developing an electrostatic latent image of an original formed on the surface of the copy paper into visible toner image, through processing devices such as a copy paper cutting device, a corona charging device, an exposure device and the like disposed along a path of the copy paper in a known manner.
There has conventionally been proposed a developing device employing the developing solution for use in the so-called electrofax type copying apparatus in Japanese Utility Model Publication Jitsukaisho 47/25039. That device includes a plurality of impellers each having a plurality of blades rotatably disposed in a developing tank for stirring the developing solution contained therein, and means for transmitting rotation of a copy paper transporting mechanism of the copying apparatus to the stirring impellers so that the impellers are rotated simultaneously with the rotation of the copy paper transporting mechanism. The device is so constructed that the stirring impellers are fixedly mounted at regular intervals on a rotatable shaft directed at right angles to the direction of advance of the copy paper for feeding part of the developing solution being stirred toward the copy paper sheet. By this arrangement, however, it is impossible to stir or agitate the toner particles settled at the bottom of the developing tank, especially at the portion immediately below a developing electrode in a short period of time, for example, within several seconds.
Another conventional developing device of similar purpose also for use in the electrofax type copying apparatus disclosed in Japanese Utility Model Publication Jitsukaisho 50/10243 includes a guide plate provided to form a developing passage for copy paper in a developing tank in such a manner that at least part of the developing passage is located below the predetermined level of the developing solution contained in the developing tank; a driving shaft rotatably and horizontally disposed in the developing tank; and a plurality of stirring vanes each having a plurality of blades and disposed below the predetermined level of the developing solution adjacent to the bottom of the developing tank. The stirring vanes are engaged for rotation with the driving shaft which is associated with a copy paper transportation mechanism of the copying apparatus. That arrangement, however, is also incapable of agitating toner particles settled below a developing electrode in a short period of time, since the stirring vanes disposed adjacent to the developing tank are located away from the position immediately below the developing electrode.
In other words, although both of the above prior art developing devices of Japanese Utility Model Publication of Jitsukaisho 47/25039 and Jitsukaisho 50/10243 aim at driving the stirring rotary members through the copy paper transportation mechanism, nothing is taken into account concerning the rapid stirring of the toner particles settled below the developing electrode, and the essential purpose of stirring for uniformly distributing toner particles within the developing tank can not be achieved due to merely local circulation of the developing solution or only limited amount of the same solution moving within the tank.
Furthermore, it should be noted that the wet developing in the electrophotography is based on the principle that charged toner particles within the organic solvent are attracted by the electric field of the electrostatic latent image formed on the copy paper, move toward the latent image through electrophoresis and consequently adhere to the surface of the latent image by the coulomb force. In the electrophotographic copying apparatus, the amount of toner particles which stick, within a limited period of time, to the surface of the copy paper passing through the developing solution, or developing density depends on the rate of movement of toner particles, i.e., the number of toner particles reaching the latent image on the surface of the copy paper within the developing solution. The larger the amount of charge of the toner particles and also the electric field strength due to charge of the latent image are, the higher is the developing density within a limited period of time. In that case, the field strength varies to a great extent not only with the surface potential of the latent image on the copy paper, but also with the distance between the charge of the latent image on the copy paper and the developing electrode opposed thereto. Such an effect is particularly conspicuous with respect to a latent image having a spreading area.
On the other hand, in the electrophotographic copying apparatus of the latent image transfer type wherein a latent image once formed on a photoreceptor surface is transferred onto a copy paper sheet, even when a latent image having a high potential (for example, in the vicinity of 1000 volts) can be formed on the photoreceptor surface, the potential of the latent image, when transferred onto the copy paper, is reduced to approximately 1/10 of the latent image on the photoreceptor, i.e., to a low potential ordinarily in the order of 100 to 150 volts.
Accordingly, when the wet developing devices designed for use in the conventional electrophotographic copying apparatus of the electrofax type, wherein a latent image of high potential in the range of 300 to 400 volts is directly formed on the photosensitive copy paper for development by such wet developing devices as those disclosed in the Japanese Utility Model Publications Jitsukaisho 47/25039 and Jitsukaisho 50/10243, are employed for the latent image of low potential as in the above described copying apparatus of latent image transfer type using the photoreceptor, the resultant developed image is so weak in density that the same hardly meets the practical purpose.
A wet developing device having satisfactory response to charge is therefore required for developing the copy paper having a low potential charge. More specifically, in order to obtain copied images of sufficient developing density from the latent image having low potential in the region of 100 volts as described above, developing solution which employs toner particles with a small amount of charge corresponding to the charge of the latent image should be adopted. At the same time, the ratio of the toner particles in such a developing solution must be increased, for example, up to approximately 30 %, which is the equivalent to approximately 4 to 6 times the ratio of toner particles in the developing solution employed in the conventional developing devices for use in the copying apparatus of the electrofax type. On the other hand, the developing electrode plate employed in the conventional copying apparatus of the electrofax type is not suitable for the above purpose, since in such prior art electrode plates, the distance between the copy paper surface and the electrode plate is too large with the slow moving rate of toner particles. This results in insufficient developing density, from which fact, employment of a pair of electrode rollers of metallic material suitably grounded and adapted to rotate in direct contact with the latent-image-formed surface of the copy paper is required for reducing the distance between the electrode and the latent image and, thus, for fully increasing the field strength. Even in the above arrangement, however, since the contact between the electrode roller and the copy paper is nearly linear contact, the region having high field strength is limited to a small portion in the vicinity where the electrode rollers immediately contact the copy paper, and consequently, the time required for the copy paper to pass such a small portion between the pair of electrode rollers tends to be reduced as compared with that required in the conventional devices employing the electrode plate. These drawbacks may be overcome by providing a plurality of pairs of electrode rollers, preferably more than three pairs, together with the marked increase of the field strength in the above described manner.
Accordingly, it is desirable that a developing device using developing solution of high toner concentration, with a plurality of electrode rollers provided therein should be employed as a wet developing device for developing the copy paper bearing thereon the electrostatic latent image of low potential. Such a wet developing device, however, still has such disadvantages that, because of the high toner powder content in the developing solution, the toner particles are likely to settle on the bottom of the developing tank and make it impossible to obtain definite copied images having the proper density. Without rectification of which defects, a wet developing device actually suitable for practical use can not be presented.
From the foregoing description it is clear that in the wet developing device especially for use in the electrophotographic copying apparatus of the latent image transfer type, the toner particles tend to settle very easily in comparison to the device employed in the copying apparatus of the electrofax type, for which tendency an effective counter-measure must be introduced.